The Kleene * means “zero or more occurrences of the immediately previous character or regular expression”.

Kleene *

The Kleene + means “one or more occurrences of the immediately preceding character or regular expression”.

Kleene +

This operator represents the "or" condition logic. Text strings at both sides of the operator represent conditions to be matched. The operator "|" is also called the pipe symbol.

E.g. /cat|dog/ -> matches either the string cat or the string dog.


Disjunction (OR) operator  | 

Enclosing patterns in parentheses makes the whole search pattern remains one string, while disjunction operator | can be placed within, to specify the variations of the search pattern.
E.g.  /gupp(y|ies)/ -> matches either the string guppy or guppies.


Parenthesis (grouping) operator ()

"*"  0 or more previous character(s) or expression(s)

"+" 1 or more previous character(s) or expression(s)

"? " 0 or 1 previous previous character(s) or expression(s)

{n}/ {n,}/ {,m}/ {n,m} exact number or number range of occurrence

Operators for counting (regular expression)

\d one digit from 0 to 9

\D one character that is not a digit

\w one word character

\W one character that is not a word character

\s one whitespace character

\S one character that is not a whitespace character 

Aliases for common ranges in regular expression

The string of characters inside the braces specifies a disjunction of characters to match.

Disjunction: []

Dash (-) can be used to specify any one character in a range.

Range: -

Anchors are special characters that anchor regular expressions to particular places in a string.

Anchors: $

The square braces can also be used to specify what a single character cannot be, by use of the caret ˆ.

Exclude: ^

The question mark /?/ means “the preceding character or nothing”.

Optional elements: ?

The simplest kind of regular expression is a sequence of simple characters.
Due to the complexity and features of regular expressions, several signs are designed to better match characters while searching.

University of Michigan - Ann Arbor

- A regular expression is "an algebraic notation for characterizing a set of strings". They are particularly useful for searching patterns in texts.
- Regular repressions helps to represent text patterns to be extracted from a document.

Regular Expression

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

A regular expression cheat sheet

https://cheatography.com/davechild/cheat-sheets/regular-expressions/

Regular expression reference

Basic Regular Expression Patterns

The order of regex operator precedence: Parenthesis > Counters > Sequences and anchors > Disjunction.

For example, the Kleene* operator applies by default only to a single character, not to a whole sequence. The whole pattern can be repeated zero or more times if we can add * after the search pattern in ().

Operator precedence hierarchy

Greedy matching: In the greedy mode (by default), regular expressions always match the largest string they can.
Non-greedy matching: The operator +? is a Kleene plus that matches as little text as possible.

Greedy/ non-greedy matching

False positive: strings that were incorrectly matched
False negative: strings that were incorrectly missed
Increasing precision and recall by fine-tuning the search patterns
(minimizing false positives or false negatives)


False positive and false negative errors for regex matching

A useful cheatsheet for regex in one picture

Regular Expression Cheatsheet

Use parentheses to store a pattern in memory.

Capture Group

Occasionally we might want to use parentheses for grouping, but don’t want to capture the resulting pattern in a register. In that case we use a non-capturing group, which is specified by putting the commands non-capturing group"?".

Non-capturing Group

There will be times when we need to predict the future: look ahead in the text to see if some pattern matches, but not advance the match cursor, so that we can then deal with the pattern if it occurs.


Lookhead Assertion

We say that the patterns are greedy in cases when regular expressions match the largest string they can.

Greedy Matching in Regular Expression

The patterns are non-greedy when regular expressions match as little text as possible. For example, the operator *? and +? both enable non-greedy matching.

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